I remember hearing about an asteroid that passed close to Earth, but we didn’t even know it was there until it had already passed us. How could an asteroid just appear out of nowhere like that if we’re always scanning the sky for them?

Don Yeomans: I believe you are referring to asteroid 2002 EM7, an object that passed 1.2 lunar distances of Earth on March 8, 2002, but was not discovered until four days later.

In this particular case, this rather small, 50-meter sized object passed Earth coming from the direction of the sun, so it was in the daylight sky, and hence unobservable with ground-based telescopes.

Objects of this size pass unnoticed within a lunar distance of Earth every few months. Many are not discovered because they are small and faint. Current survey telescopes often cannot pick them up until they approach the Earth, so the time window for discovery is relatively short.

The asteroid 2002 EM7 was discovered when it became observable in the nighttime sky. While close to the Earth, such objects have a distinctive apparent motion against the stellar background, and it is this motion that allows their discovery by wide-field survey telescopes. Once discovered, these objects are normally observed for a long time – long enough that an accurate orbit can be determined and predictions can be made about their future motions in space. Then we can determine if such objects will come close to Earth again.

Alan Harris: Such asteroids pass near the Earth many times before they impact. The objective of the Spaceguard Survey is to discover asteroids as they pass by the Earth on one of those prior occasions, which is exactly what happened with 2002 EM7. Whether the asteroid was discovered while approaching or receding is not a major issue.

In movies like "Armageddon" and "Deep Impact," nuclear detonations are used to try to divert near-Earth objects. But I’ve heard this really is not a good option, because it would break the asteroid into many pieces and increase our odds of being hit. What is your opinion?

Clark Chapman: The advantage of using nuclear weapons to destroy asteroids is that they are our most powerful devices by far. But the disadvantages are many. In particular, the more we learn about asteroids and comets, the more we realize that they are incredibly fragile. Most asteroids larger than a few hundred meters across are now thought to be "rubble piles" — collections of rocks, boulders, and "mountains" simply resting against each other, loosely held together by the tenuous gravitational field of the ensemble.

Any sudden force applied to such an object would likely tear it apart into a swarm of objects. The total impacting energy of the swarm would be the same as the original asteroid, but spread out across the Earth’s surface. In any case, once you disrupt a comet or asteroid into many different chunks, you’ve lost all ability to affect what happens next. In short, it is a very bad idea.

How would a comet impact differ from an asteroid impact? Or would there not be much difference among objects of the same size and velocity?

Impact of the Tunguska explosion is visible even after 90 years.Credit: Galena HS

Alan Harris: The main difference applies to objects just barely big enough to penetrate the Earth’s atmosphere, like the Tunguska event. We can conclude the Tunguska cosmic body was a "hard stone" asteroidal object, because if it had been soft and fluffy it would have exploded at a much higher altitude than it did. Likewise, an iron body would have hit the ground and produced a crater about the size of Meteor Crater in Arizona.

But for larger objects, the nature of the impactor hardly matters. A 1 kilometer-diameter object will punch right through the atmosphere regardless of its velocity or composition. So if it is that large, there is not much difference in effect between iron, rock, or a snowball of the same mass.

Suggested strategies to divert asteroids include an electromagnetic machine that hurls dirt from the surface, an orbiting parabolic mirror to heat up the surface and create a plume of vaporized material, or the low-tech strategy to ‘paint-it-black,’ where the asteroid is coated so solar heating would divert the asteroid. (Or alternatively, to strip a thin layer from the surface — the newly exposed colors would change the asteroid’s thermal properties enough to move it.) What do you think of these options, and what would be your favored mitigation strategy if we only had three to five years of advance warning?

Clark Chapman: The three approaches suggested in the question are potentially viable. I see problems with the practicality of the electromagnetic "mass driver" concept, but it is possible that a scheme could be developed with more research. Three to five years warning is rather short, though, so there would be little time to develop any technologies that don’t already exist.

I am very dubious about the "paint it black" concept, especially in the context of a short warning time. The so-called Yarkovsky Effect, in which a spinning body is slowly accelerated due to asymmetric re-radiation of sunlight impinging on the body, is very weak and probably would be effective only over a much longer duration. And having just gone through a terrible saga last year getting my house painted, I’m skeptical about the practicality of "painting" or "stripping" the surface of an asteroid!

The best technique might depend upon the size and nature of the threatening body. I like the concept being explored by the B612 Project: attach a low-thrust rocket (a long-acting, nuclear-powered plasma engine) to the asteroid, de-spin it, and then move it away from its Earth-impact trajectory. But further development and integration of the engineering concepts are needed.

Could we reduce the danger from planet-killer sized asteroids by locating all the troublesome objects and mining them out of existence? We would get valuable materials and also save the planet.

An aerial view of Meteor Crater, Arizona.Credit: Jim Hurley, 1978

Alan Harris: There are two problems with this concept. First, "all the troublesome objects" at present is zero, and is likely to remain so. We don’t expect to find any asteroids on a collision course with the Earth. If we did find one, "mining it out of existence" would be a vastly greater enterprise than simply deflecting it off of a collision course.

There is a common misconception of the utility of space resources. With present technology, it makes no sense to go into space for resources to bring back to Earth. The only sensible utility of mining asteroids is for resources to be used in space — that is, to reduce the amount of mass that must be thrown up into space against the Earth’s gravity. We might contemplate mining the offending asteroid to gain fuel to deflect it, or for mass to run a mass driver, but not to bring stuff back home.

How many times in recorded history has a significant asteroid or comet impact occurred? You mentioned the event in China where 10,000 people may have been killed by asteroids. Is that the most deadly asteroid event that has occurred for humans?

Benny Peiser: We have no idea how many significant impacts have occurred during the last 10,000 years. While we have a number of historical records that appear to refer to cosmic impacts, many of these accounts are too ambiguous to give us any reliable information. This predicament is also true for the various reports regarding the alleged impact disaster in China during the 15th century.

Clark Chapman: A paper was published about half-a-dozen years ago that interpreted ancient Chinese records in terms of meteoroid impacts. I found essentially all of the instances in that paper to be *in*credible. A case of stones raining down on an army violated one of the most characteristic aspects of meteoroid falls: all the stones were interpreted to have been about the same size. Instead, real debris from outer space — whether broken up in outer space or in an atmospheric explosion — forms a "power-law size distribution." There are a few big objects, and increasing numbers of small objects at ever-smaller sizes.

Eyewitness reports in modern society are notoriously unreliable, and reports from different cultures in ages long past are even more so. Presumably these historical accounts refer to something, but I doubt that most (or any) of them have much to do with impacts.

Benny Peiser: Unless you can verify the existence of an unambiguously dated impact crater, historical records and eyewitness accounts are regarded as insufficient evidence for an impact. We even find it difficult to believe the descriptions of experienced astronomers, such Leon Stuart, who claimed to have observed — and indeed photographed — a lunar impact in 1953.

This is a real dilemma since only around 5 percent of terrestrial impacts produce a hypervelocity impact crater. For every crater-producing multi-megaton impact, we can expect about 10 atmospheric or oceanic impacts that fail to produce a "smoking gun." In other words, the vast majority of small asteroids striking the Earth explode in the atmosphere. In rare cases, as happened in Tunguska, atmospheric impacts can cause considerable destruction on the Earth’s surface without leaving any compelling fingerprints (like an impact crater).

It is striking, nevertheless, that significantly more terrestrial impact craters exist that date to the Holocene (the last 10,000 years) than we have historical impact reports for. It seems the vast majority of historical impacts went unnoticed. Another possibility is that impact reports were censored by religious authorities who were concerned about the demoralizing implications of these "divine interventions."

I read a story about an asteroid that is expected to pass close to Earth sometime around the year 2016, and the temperature of the Earth would be raised to 50 degrees Celsius (122 F) at the moment it passes by. Is that possible?

The ion-propelled Deep Impact 1 spacecraft.Credit: NASA

Alan Harris: No. The topic of cosmic impacts brings out a lot of crackpot claims, and this appears to be one of them. Even an asteroid "the size of Texas" (as breathlessly declared in the movie "Armageddon") would have no discernable effect passing close to the Earth — as long as it didn’t hit.

A proposal recently submitted to the European Space Agency has a fleet of five mini-probes targeting an asteroid considered potentially dangerous. Once in space, the probes would use ion propulsion engines that provide thrust by shooting out a stream of electrically charged particles. How well do you think this would work?

Don Yeomans: The European Space Agency has received several recent proposals to utilize spacecraft to either discover near-Earth objects or study their compositions and structures. None of these proposals would actually try to use the very low thrust of ion propulsion engines to attempt an asteroid deflection. One of these proposals, called SIMONE, would use five low-cost spacecraft to fly by, or rendezvous with, a number of near-Earth objects to gain an understanding of their physical nature. This information would be invaluable should we one day have to deflect an Earth-threatening object.

The method employed in a future asteroid deflection attempt would depend upon a detailed knowledge of the composition, mass, rotation, and structure of the threatening near-Earth object. Although probably not appropriate for deflecting a near-Earth object, low-thrust, ion-drive spacecraft are very efficient in terms of propulsion. They are also very reliable. The Deep Space 1 ion-drive spacecraft has been operating successfully in space since October of 1998.

Could you describe what the public reaction to the 1908 Tunguska impact was like at the time? Do you think the public today would respond differently?

Benny Peiser: The atmospheric impact over the Tunguska region in Siberia was witnessed by many thousands of people and felt over an area of more than 1,000 miles in radius. Many native Tungus hunters were fairly close to ground zero, and some of them witnessed the large-scale slaughter of their deer herds as a result of the blast. Apparently, one or two hunters were killed by the explosion. Reminiscent of religious leaders who blame natural disasters on a vengeful deity, Tungus shamans told their people that disobedience had brought divine calamity upon themselves.

Further away from the epicenter, the disaster also was witnessed with trepidation. Just days after the impact, many Russian newspapers reported the huge explosion. A newspaper from Irkutsk, for example, described how peasants in the village of Nizhne-Karelinsk (200 miles from ground zero) "saw a body shining very brightly with a bluish white light. When the shining body approached the ground it seemed to be pulverized, and in its place a huge cloud of black smoke was formed and a loud crash — not like thunder, but as if from the fall of large stones or from gunfire — was heard. All the buildings shook and at the same time, a forked tongue of flame broke through the cloud. Everyone thought that the end of the world was approaching."

Despite many similar reports and eyewitness accounts, newspapers and scientists discarded the whole incident, claiming that such bizarre stories were unsound and unreliable. As a result of this general disbelief, it took some 20 years before the first scientific excursion reached ground zero to investigate the causes of the catastrophe.

And what about today? It is unlikely that we will encounter another Tunguska event in the near future. Impacts in the 10-megaton range probably happen only once every 500 to 1,000 years. But for argument’s sake, let us contemplate how the public might respond if another Tunguska-type impact happened tomorrow.

For a start, the reaction of the public fundamentally will depend on the location, extent, and destruction of the impact. In all likelihood, another Tunguska event would occur over an unpopulated or scarcely inhabited region of the world. However, in the unlikely event of fatalities, the global uproar could be substantial. In such a case, 9/11 would look like an insignificant security failure. The blame game would be brutal, and I would certainly not like to be in the shoes of those who had advised the government that small impacts were negligible.